5w-30 or 10W-60 in an M54 @ 140 000 km now ?

Hello,

Until mid 2008, my BMW dealer used to put Castrol oil (SAE 5W30 LL04) in
my Z3 (M54 Engine).
Before last November, BMW rationalized their stocks, restricting the
varieties.
At the last Oil Service, they put a thinner oil, rebranded "Original BMW
Quality Longlife-04 SAE 5W-30", made in Germany exclusively for BMW AG...
Basically it's the same quality level but more fluid in hot temps.

The result is that the oil consumption increased from 0.32 to 0.57 l/1000 km
(when new it was 0.28 l/1000). Although the car had 132 000 km on it,
immediately after the oil service, It began to burn oil. Now, the black
deposit at the exhaust is quite solid, barely hard to be removed.
Appearently no leak, at least no stain on the garage floor tiles.
I always had a rather high oil consumption with my different BMWs, mainly
because I'm heavily using engine braking but it's not the reason of the
sudden increase. I suspect that the viscosity of this new oil is the first
thing to focus on.

The questions a
- Although BMW doesn't recommends 10W-60 (reserved to the M3s), does anybody
have feedback using this grade in his M54 ?
- Besides the price effect on the wallet thickness, what would be the
drawbacks, especially at very low temperatures (0°c) ? Uh! I'm living on the
French Riviera ;-)

Thanks for your help.